1. Field of the Invention
This invention relates generally to the field of piston-type compressors, such as the compressors that are commonly used in vehicle air conditioning systems. More specifically, this invention relates to a system that has been developed in order to minimize the shock and vibration that is typically associated with start-up operation in this type of compressor.
2. Description of the Related Technology
Generally, in a fixed-volume, twin-head piston compressor of the type that is used in automotive air conditioning systems, twin-head pistons are contained in the cylinder bores that are formed in a cylinder block. An intake chamber and a discharge chamber are formed in the front housing unit and the rear housing unit on a partitioned basis. The cooling medium gas inside the intake chamber is drawn in and compressed by the reciprocating motion of the pistons and is discharged into the discharge chamber.
In the twin-head piston compressor, the discharge holes are formed on the valve plate in correspondence with the cylinder bores. Fixed discharge valves, which are provided in correspondence with fixed discharge holes, are closed when the cooling medium gas is drawn in and are opened when the gas is discharged.
In such a compressor, when by engaging the clutch the compressor is mechanically connected to an external drive source, such as the vehicle engine, and the reciprocal motion of the pistons begins, the compression load on the compressor increases suddenly. In a vehicle-mounted compressor, the sudden change in compression load is transmitted to the external drive source, such as a vehicle engine, as a change in load torque. This can cause a change in the rpm of the vehicle engine. The change in rpm is called an on-off shock and imparts an unpleasant sensation to the passenger.
Moreover, when the compressor is started, a compression force sometimes acts on the liquid coolant that has been deposited in the cylinder bore. When the compressor assumes the liquid-compressed state, a high compression load acts upon the pistons and generates shock-like vibrations and noise.
In order to solve these problems, the present applicant has previously proposed a compressor equipped with a startup shock, an example of which can be found in Japanese Kokai patent S61-72885 (1986). In this compressor, moving discharge valves are provided in correspondence to the discharge holes located on the rear valve plate. The moving discharge valves operate in conjunction with the motion of the spool; they are moved on a switchable basis between the operating position, in which the valves come into contact with the discharge holes, and the non-operating position, in which the valves are at a distance from the discharge holes. A control chamber is provided on the back of the spool. A first electromagnetic valve is located on the pressure supply passage that connects the control chamber to the discharge pressure area. Similarly, a second electromagnetic valve is located on another pressure supply duct that connects the control chamber to the suction pressure area. When the compressor is started, the activation signal opens and closes, respectively, the first and second electromagnetic valves, connects the control chamber to the discharge pressure area, and shuts off the connection to the suction pressure area. Consequently, the spool is moved by the discharge pressure supplied to the control chamber against the energizing force of the spring. This places the moving discharge valves, located in the non-operating position, in the operating position. When the compressor is stopped, the stop signal opens and closes, respectively, the first and second electromagnetic valves, connects the control chamber to the suction pressure area, and shuts off the connection to the discharge pressure area. Consequently, the pressure inside the control chamber is released to the suction pressure area and decreases. When the spool is moved by the energizing force of the spring, the moving discharge valves are moved to the non-operating position.
Thus, by placing the moving discharge valves in the non-operating position when the compressor is stopped, the normal compression operation is not performed in the compression chamber with which the moving discharge valves are associated for several seconds from the time the compressor is started until the moving discharge valves are transferred into the operating position. Consequently, the compression load that occurs during the activation of the compressor increases only gradually. This reduces both the unpleasant sensation that occurs when the clutch is engaged and the generation of noise and vibrations that stems from the compression of the liquid coolant.
The technology described above requires the provision of electromagnetic valves on the pressure supply passage as well as a control computer that controls the opening and closing operations of the electromagnetic valves according to compressor start/stop information. This adds complexity to the structure of the startup shock absorber and the compressor, thus increasing manufacturing costs.
A need exists for an improved piston-type compressor that is capable of reducing the compression load that acts on the pistons during start-up operation that is relatively inexpensive to manufacture and is effective regardless of the initial position of the pistons during start-up.